1. Field of the invention
This invention relates generally to machines for making bags or the like from a continuous web or sheet of film material, such as plastic. More particularly, the invention relates to an apparatus and method for detecting perforations imparted onto the web at regularly spaced intervals.
2. Description of Related Art
Bag making machines are well known in the art. These machines typically include a sealing drum for imparting transverse seals onto the web material to thereby define individual plastic bags, a folding board section for folding the bags one or more times in the longitudinal direction, and a perforator for imparting series of perforations to the web adjacent the seals so that the individual bags may subsequently be separated from the web. After passing through the bag making machine, the web is usually directed through a bag winding machine, wherein the sealed and perforated web is rolled up into individual rolls consisting of a predetermined number of bags. Some prior art bag winding machines comprise a web separating apparatus for either separating the web at a particular series of perforations after a predetermined number of bags has been rolled up or separating the web at each series of perforations. Such a bag winding machine is disclosed in U.S. Pat. No. 5,161,793 issued to Lotto et al., in which the web separating apparatus comprises a pair of infeed rolls and a pair of nip rolls located a short distance downstream of the infeed rolls. The nip rolls are driven at a speed slightly greater than the infeed rolls and are normally not in contact with each other. When it is desired to separate the web at a particular series of perforations, the nip rolls are brought into contact with each other and the portion of the web caught between the nip rolls is pulled away from the portion of the web caught between the infeed rolls until the web consequently separates at the series of perforations. In addition, in the type of bag winding machine disclosed in Lotto et al., as each bag is being separated from the web, it passes into an interleaving or overlap forming zone, wherein the trailing edge of a leading bag is overlapped with the leading edge of the following bag. The interleaving zone comprises an air flow delivery manifold and a vacuum manifold operating in conjunction with either another air flow delivery manifold, a vacuum box or a mechanical loop former to direct the trailing edge of the leading bag and the leading edge of the following bag into an overlapping arrangement.
In prior art bag winding machines such as described above, the actuation of the web separating apparatus and the various interleaving devices must be precisely timed so that the web can be processed at high speeds. In addition, the actuation of these elements is dependent upon the position of the series of perforations. For example, the web separating apparatus will be actuated when the series of perforations is located between the infeed rolls and the nip rolls. Thus, knowing the positions of the series of perforations as the web passes through the bag winding machine is very important to the proper operation of the bag winding machine. In some prior art bag making systems, the position of a particular perforation in the bag winding machine can be estimated, provided the position of the perforation in the bag making machine and the distance between the bag making machine and the bag winding machine are known. However, when the distance between the bag making machine and the bag winding machine is great, as is often the case in dual lane systems wherein two webs are fed from a single bag making machine to two separate bag winding machines, the web can stretch and billow to such an extent that the position of the series of perforations cannot accurately be determined from the distance between the bag making machine and the bag winding machine.
One prior art device addresses this problem by attempting to detect the series of perforations as the web passes into the bag winding machine. This device uses electric leads on each side of the web to pass an electric arc through the perforation in the series and thereby complete a circuit each time a series of perforations passes by. The disadvantages of this device are that the web must travel relatively slowly due to switching frequency limitations (around 8 Hz) and, even at this slow speed, the individual perforations may not always provide the opening required to pass the electric arc. Therefore, both the production rates and the detection accuracy are low in systems employing this device.